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Transitor as heater

Started by George Herold February 13, 2014
Transistor as heater/ temp sensor.  
So in looking for a small heater one idea is to use a transistor as heater.  
And to take the idea one step further to use the same transistor (sequentially)
As first a heater then a temp sensor.  
So the first question is then how to use it as a heater.  I was thinking of controlling the current 
At some fixed voltage.. something like this,

https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG

But other ideas would be welcome.  One semi-crazy idea, I'm using the transistor as a temp sensor with the c-b shorted.  (diode connected transistor.)  Could I just push a bunch more current through it for a heater.  

The second problem I see with the heater idea is how to do the switching from heater to temp sensor.  Do I use relays or analog switches?  
(I'm off to look into anaolg switches.)

Thanks again,
George H.
On Thu, 13 Feb 2014 07:26:01 -0800, George Herold wrote:

> Transistor as heater/ temp sensor. > So in looking for a small heater one idea is to use a transistor as > heater. > And to take the idea one step further to use the same transistor > (sequentially) > As first a heater then a temp sensor. > So the first question is then how to use it as a heater. I was thinking > of controlling the current At some fixed voltage.. something like this, > > https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG > > But other ideas would be welcome. One semi-crazy idea, I'm using the > transistor as a temp sensor with the c-b shorted. (diode connected > transistor.) Could I just push a bunch more current through it for a > heater. > > The second problem I see with the heater idea is how to do the switching > from heater to temp sensor. Do I use relays or analog switches? > (I'm off to look into anaolg switches.) > > Thanks again, > George H.
Using it as a temperature sensor would be complicated by the fact that, due to its recent service as a heater, it would be warmer than its surroundings. Since transistors are cheaper than relays or analog switches, why not use one for heating and another for temperature sensing? -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On Thu, 13 Feb 2014 09:51:06 -0600, Tim Wescott wrote:

> On Thu, 13 Feb 2014 07:26:01 -0800, George Herold wrote: > >> Transistor as heater/ temp sensor. >> So in looking for a small heater one idea is to use a transistor as >> heater. >> And to take the idea one step further to use the same transistor >> (sequentially) >> As first a heater then a temp sensor. >> So the first question is then how to use it as a heater. I was >> thinking of controlling the current At some fixed voltage.. something >> like this, >> >> https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG >> >> But other ideas would be welcome. One semi-crazy idea, I'm using the >> transistor as a temp sensor with the c-b shorted. (diode connected >> transistor.) Could I just push a bunch more current through it for a >> heater. >> >> The second problem I see with the heater idea is how to do the >> switching from heater to temp sensor. Do I use relays or analog >> switches? >> (I'm off to look into anaolg switches.) >> >> Thanks again, >> George H. > > Using it as a temperature sensor would be complicated by the fact that, > due to its recent service as a heater, it would be warmer than its > surroundings. > > Since transistors are cheaper than relays or analog switches, why not > use one for heating and another for temperature sensing?
And since, watt-for-watt, resistors are cheaper than transistors, why not use a transistor to hard-switch a resistor for your "heater"? -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
On 2/13/2014 10:26 AM, George Herold wrote:
> Transistor as heater/ temp sensor. > So in looking for a small heater one idea is to use a transistor as heater. > And to take the idea one step further to use the same transistor (sequentially) > As first a heater then a temp sensor. > So the first question is then how to use it as a heater. I was thinking of controlling the current > At some fixed voltage.. something like this, > > https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG > > But other ideas would be welcome. One semi-crazy idea, I'm using the transistor as a temp sensor with the c-b shorted. (diode connected transistor.) Could I just push a bunch more current through it for a heater. > > The second problem I see with the heater idea is how to do the switching from heater to temp sensor. Do I use relays or analog switches? > (I'm off to look into anaolg switches.) > > Thanks again, > George H. >
Not crazy at all--you can get better loop bandwidth that way. It's thermal conduction that's slow. Put a voltage divider from collector to emitter, with the base connected to the middle. That makes your transistor an n*V_VBE voltage drop, for some convenient value of n. Use a resistor in parallel with a high side switch, and measure the collector-emitter voltage between pulses. You do have to use enough sense current that the sensor/heater device doesn't run out of gain. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Thursday, February 13, 2014 10:51:06 AM UTC-5, Tim Wescott wrote:
> On Thu, 13 Feb 2014 07:26:01 -0800, George Herold wrote: >
> > > Using it as a temperature sensor would be complicated by the fact that, > due to its recent service as a heater, it would be warmer than its > surroundings.
Yeah well I'm going to have to pulse the heat and then wait for it to diffuse (or whatever heat does) into the sample. So I've got a pulse and then a wait time for the temperature to settle down. No matter what I use I need good thermal contact between the sample and my heater/ temp sensor, gizmo (sometimes called the addendum those that do heat capacity measurements.)
> > > > Since transistors are cheaper than relays or analog switches, why not use > one for heating and another for temperature sensing?
Yeah I could do that too. Cheap is not really the issue here. I want low mass/ easy to mount/ good themal contact. (yeah I know, pick two of the three :^) George H.
> > > > -- > > Tim Wescott > > Control system and signal processing consulting > > www.wescottdesign.com
On Thursday, February 13, 2014 10:26:01 AM UTC-5, George Herold wrote:
> Transistor as heater/ temp sensor. > > So in looking for a small heater one idea is to use a transistor as heater. > > And to take the idea one step further to use the same transistor (sequentially) > > As first a heater then a temp sensor. > > So the first question is then how to use it as a heater. I was thinking of controlling the current > > At some fixed voltage.. something like this, > > > > https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG > > But other ideas would be welcome. One semi-crazy idea, I'm using the transistor as a temp sensor with the c-b shorted. (diode connected transistor.) Could I just push a bunch more current through it for a heater. > > The second problem I see with the heater idea is how to do the switching from heater to temp sensor. Do I use relays or analog switches? > > (I'm off to look into anaolg switches.) > > Thanks again, > > George H.
Using the transistor as sensor and heater sounds like a fine idea-- you can't beat the thermal coupling. V+ -+- | Q1 --+ |<' | --| .-. |\ | | R1 | | | | '-' | | '--+ | .-. | | R2 | | '-' | .---+----> | | | |/ '-| Q2 |>. | | === Forcing high current into the transistor might cause undesirable heating elsewhere--caveat emptor. Ye Olde d(Vbe)/d(Ic) temperature trick (above) might be handy. Cheers, James Arthur
On Thursday, February 13, 2014 10:52:05 AM UTC-5, Tim Wescott wrote:
> On Thu, 13 Feb 2014 09:51:06 -0600, Tim Wescott wrote: >
,snip> surroundings.
> > > > > Since transistors are cheaper than relays or analog switches, why not > > use one for heating and another for temperature sensing? > > And since, watt-for-watt, resistors are cheaper than transistors, why not > use a transistor to hard-switch a resistor for your "heater"?
Yeah I've done that in the past. (put both the pass element and resistor on the thing to be heated.) This is all about small and low mass. I've got a little clamp sketeched out. A couple of 2-56 screws (0.4g) and what looks to be about 1.5 grams of copper on the bottom part and maybe 1/2 gram of G-10 for the flexible top of the clamp. George H.
> > > -- > > Tim Wescott > > Control system and signal processing consulting > > www.wescottdesign.com
On Thursday, February 13, 2014 10:56:15 AM UTC-5, Phil Hobbs wrote:
> On 2/13/2014 10:26 AM, George Herold wrote: >=20 > > Transistor as heater/ temp sensor. >=20 > > So in looking for a small heater one idea is to use a transistor as hea=
ter.
>=20 > > And to take the idea one step further to use the same transistor (seque=
ntially)
>=20 > > As first a heater then a temp sensor. >=20 > > So the first question is then how to use it as a heater. I was thinkin=
g of controlling the current
>=20 > > At some fixed voltage.. something like this, >=20 > > >=20 > > https://www.dropbox.com/s/plcgg2rgnh0byt7/Tran-heat.JPG >=20 > > >=20 > > But other ideas would be welcome. One semi-crazy idea, I'm using the t=
ransistor as a temp sensor with the c-b shorted. (diode connected transist= or.) Could I just push a bunch more current through it for a heater.
>=20 > > >=20 > > The second problem I see with the heater idea is how to do the switchin=
g from heater to temp sensor. Do I use relays or analog switches?
>=20 > > (I'm off to look into anaolg switches.) >=20 > > >=20 > > Thanks again, >=20 > > George H. >=20 > > >=20 > Not crazy at all--you can get better loop bandwidth that way. It's=20 > thermal conduction that's slow. Put a voltage divider from collector to=
=20
> emitter, with the base connected to the middle. That makes your=20 > transistor an n*V_VBE voltage drop, for some convenient value of n. Use=
=20
> a resistor in parallel with a high side switch, and measure the=20 > collector-emitter voltage between pulses.
Ahh, that's the Vbe multiplier that you see in push-pull stages. =20 A very interesting idea! Thanks. =20 One issue I have with using the transistor in this way (as well as the simp= le diode connected transistor) is that as I pulse the transistor it's going= to warm up. Vbe will change, and so the amount of heat delivered to the = sample will change. (during the pulse) I'd have to monitor the Vce voltage= and integrate (or something) to get the energy. That's not impossible, bu= t it's not as clean as I*V*time for a resistor, or wrapping an opamp around= the transistor to keep the current constant. =20 It does make another part of life easier though...=20 George H.
>=20 >=20 > You do have to use enough sense current that the sensor/heater device=20 >=20 > doesn't run out of gain. >=20 >=20 >=20 > Cheers >=20 >=20 >=20 > Phil Hobbs >=20 >=20 >=20 >=20 >=20 > --=20 >=20 > Dr Philip C D Hobbs >=20 > Principal Consultant >=20 > ElectroOptical Innovations LLC >=20 > Optics, Electro-optics, Photonics, Analog Electronics >=20 >=20 >=20 > 160 North State Road #203 >=20 > Briarcliff Manor NY 10510 >=20 >=20 >=20 > hobbs at electrooptical dot net >=20 > http://electrooptical.net
On Thu, 13 Feb 2014 07:26:01 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>Transistor as heater/ temp sensor.
Use a separate heater and sensor. The problem is that you're trying to regulate the temperature of some attached device, such as a crystal, and NOT the temperature of the heater. If you simply attached a crystal to your proposed stabilized heater, the transistor junction temperature would be stabilized, but the temperature of the crystal would be susceptible to ambient changes. In effect, you would have a temperature gradient starting with the stabilized heater, and ending with room temperature, with the crystal somewhere in between. You could insulate the entire affair to minimize this, but I think that will burn up all the alleged cost savings of having everything in one package. If you look at what's inside a typical OCXO, you'll usually find the crystal sandwiched between a TO-3 transistor heater and a thermistor. Sometimes, there's a heater wire wrapped around the crystal can. The important point is that the thermistor is on the other side of the heater. The idea is to make the crystal can part of the temp sensor and thus stabilize the crystal can temperature, not the heater or junction temperature. You didn't mention what you were heating. If it has a large mass, you're also going to have thermal lag and stabilization issues. If it's truly massive, it can take a long time to stabilize. If you need extreme stability, mechanical considerations tend to be more important than electronics. -- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Den torsdag den 13. februar 2014 17.44.59 UTC+1 skrev George Herold:
> On Thursday, February 13, 2014 10:52:05 AM UTC-5, Tim Wescott wrote: > > > On Thu, 13 Feb 2014 09:51:06 -0600, Tim Wescott wrote: > > > > > ,snip> > > > > surroundings. > > > > > > > > > > > Since transistors are cheaper than relays or analog switches, why not > > > > use one for heating and another for temperature sensing? > > > > > > And since, watt-for-watt, resistors are cheaper than transistors, why not > > > use a transistor to hard-switch a resistor for your "heater"? > > Yeah I've done that in the past. (put both the pass element and resistor on the thing to be heated.) This is all about small and low mass. > > I've got a little clamp sketeched out. A couple of 2-56 screws (0.4g) and what looks to be about 1.5 grams of copper on the bottom part and maybe 1/2 gram of G-10 for the flexible top of the clamp. >
how about a dual fet, available in anything from d-pack to to-220 one fet as heater, other fet body diode as temp sensor? -Lasse